Namespace simplification pt4 (final)

[mlw]

The style guide prefers that a projects do not overly nest namespace names, and note that unless necessary, a single top-level namespace per project is the most effective strategy to limit exposure to collisions.

This is the 4th and final part of a series of PRs to convert namespaces. This is being done piecemeal to keep PR sizes manageable. This PR converts the namespaces:

• santa::santad::event_providers
• santa::santad

Note: The namespace santa::santad::process_tree is being left alone for now as that piece will soon be split off to another standalone repo.

References:

• Namespaces style guide
• Namespace names style guide
• Tip of the Week #130: Namespace Naming
• Part 1: Adopt namespace naming guidelines - part 1 #1384
• Part 2: Namespace simplification pt2 #1385
• Part 3: Namespace simplification pt3 #1386